Nematodes (derived from the Greek word for thread) are active, flexible, elongate, organisms that live on moist surfaces or in liquid environments, including films of water within soil and moist tissues within other organisms. While only 20,000 species of nematode have been identified, it is estimated that 40,000 to 10 million actually exist. Some species of nematodes have evolved to be very successful parasites of both plants and animals and are responsible for significant economic losses in agriculture and livestock and for morbidity and mortality in humans (Whitehead (1998) Plant Nematode Control. CAB International, New York).
Nematode parasites of plants can inhabit all parts of plants, including roots, developing flower buds, leaves, and stems. Plant parasites are classified on the basis of their feeding habits into the broad categories: migratory ectoparasites, migratory endoparasites, and sedentary endoparasites. Sedentary endoparasites, which include the root knot nematodes (Meloidogyne) and cyst nematodes (Globodera and Heterodera) induce feeding sites and establish long-term infections within roots that are often very damaging to crops (Whitehead, supra). It is estimated that parasitic nematodes cost the horticulture and agriculture industries in excess of $78 billion worldwide a year, based on an estimated average 12% annual loss spread across all major crops. For example, it is estimated that nematodes cause soybean losses of approximately $3.2 billion annually worldwide (Barker et al. (1994) Plant and Soil Nematodes: Societal Impact and Focus for the Future. The Committee on National Needs and Priorities in Nematology. Cooperative State Research Service, US Department of Agriculture and Society of Nematologists). Several factors make the need for safe and effective nematode controls urgent. Continuing population growth, famines, and environmental degradation have heightened concern for the sustainability of agriculture, and new government regulations may prevent or severely restrict the use of many available agricultural anthelmintic agents.
The application of chemical nematicides remains the major means of nematode control. However, in general, chemical nematicides are highly toxic compounds known to cause substantial environmental impact and are increasingly restricted in the amounts and locations in which they can be used. For example, the soil fumigant methyl bromide which has been used effectively to reduce nematode infestations in a variety of specialty crops, is regulated under the U.N. Montreal Protocol as an ozone-depleting substance and is scheduled for elimination in 2005 in the US (Carter (2001) California Agriculture, 55(3): 2). It is expected that strawberry and other commodity crop industries will be significantly impacted if a suitable replacement for methyl bromide is not found. Similarly, broad-spectrum nematicides such as Telone (various formulations of 1,3-dichloropropene) have significant restrictions on their use because of toxicological concerns (Carter (2001) California Agriculture, Vol. 55(3): 12-18).
The macrocyclic lactones (e.g., avermectins and milbemycins), as well as delta-endotoxins from Bacillus thuringiensis (Bt), are chemicals that in principle provide excellent specificity and efficacy which should allow environmentally safe control of plant parasitic nematodes. Unfortunately, in practice, these two nematicidal agents have proven less effective in agricultural applications against root pathogens. Although certain avermectins show exquisite activity against plant parasitic nematodes these chemicals are hampered by poor bioavailability due to their light sensitivity, tight binding to soil particles and degradation by soil microorganisms (Lasota & Dybas (1990) Acta Leiden 59(1-2): 217-225; Wright & Perry (1998) Musculature and Neurobiology. In: The Physiology and Biochemistry of Free-Living and Plant-parasitic Nematodes (eds R. N. Perry & D. J. Wright), CAB International 1998). Consequently despite years of research and extensive use against animal parasitic nematodes, mites and insects (plant and animal applications), macrocyclic lactones (e.g., avermectins and milbemycins) have never been commercially developed to control plant parasitic nematodes in the soil.
Bt delta endotoxins must be ingested to affect their target organ, the brush border of midgut epithelial cells (Marroquin et al. (2000) Genetics. 155(4): 1693-1699). Consequently they are not anticipated to be effective against the dispersal, non-feeding, juvenile stages of plant parasitic nematodes in the field. Because juvenile stages only commence feeding when a susceptible host has been infected, nematicides may need to penetrate the plant cuticle to be effective. Transcuticular uptake of a 65-130 kDa protein—the size of typical Bt delta ends toxins—is unlikely. Furthermore, soil mobility is expected to be relatively poor. Even transgenic approaches are hampered by the size of Bt delta toxins because delivery in planta is likely to be constrained by the exclusion of large particles by the feeding tubes of certain plant parasitic nematodes such as Heterodera (Atkinson et al. (1998) Engineering resistance to plant-parasitic nematodes. In: The Physiology and Biochemistry of Free-Living and Plant-parasitic Nematodes (eds R. N. Perry & D. J. Wright), CAB International 1998).
Fatty acids are another class of natural compounds that have been investigated as alternatives to the toxic, non-specific organophosphate, carbamate and fumigant pesticides (Stadler et al. (1994) Planta Medica 60(2): 128-132; U.S. Pat. Nos. 5,192,546; 5,346,698; 5,674,897; 5,698,592; 6,124,359). It has been suggested that fatty acids derive their pesticidal effects by adversely interfering with the nematode cuticle or hypodermis via a detergent (solubilization) effect, or through direct interaction of the fatty acids and the lipophilic regions of target plasma membranes (Davis et al. (1997) Journal of Nematology 29(4S): 677-684). In view of this predicted mode of action it is not surprising that fatty acids are used in a variety of pesticidal applications including herbicides (e.g., SCYTHE by Dow Agrosciences is the C9 saturated fatty acid pelargonic acid), bactericides, fungicides (U.S. Pat. Nos. 4,771,571; 5,246,716), and insecticides (e.g., SAFER INSECTICIDAL SOAP by Safer, Inc.).
The phytotoxicity of fatty acids has been a major constraint on their general use in post-plant agricultural applications (U.S. Pat. No. 5,093,124) and the mitigation of these undesirable effects while preserving pesticidal activity is a major area of research. Post-plant applications are desirable because of the relatively short half-life of fatty acids under field conditions.
The esterification of fatty acids can significantly decrease their phytotoxicity (U.S. Pat. Nos. 5,674,897; 5,698,592; 6,124,359). Such modifications can however lead to loss of nematicidal activity as is seen for linoleic, linolenic and oleic acid (Stadler et al. (1994) Planta Medica 60(2): 128-132) and it may be impossible to completely decouple the phytotoxicity and nematicidal activity of pesticidal fatty acids because of their non-specific mode of action. Perhaps not surprisingly, the nematicidal fatty acid pelargonic acid methyl ester (U.S. Pat. Nos. 5,674,897; 5,698,592; 6,124,359) shows a relatively small “therapeutic window” between the onset of pesticidal activity and the observation of significant phytotoxicity (Davis et al. (1997) J Nematol 29(4S): 677-684). This is the expected result if both the phytotoxicity and the nematicidal activity derive from the non-specific disruption of plasma membrane integrity.
Ricinoleic acid, the major component of castor oil, has been shown to have an inhibitory effect on water and electrolyte absorption using everted hamster jejunal and ileal segments (Gaginella et al. (1975) J Pharmacol Exp Ther 195(2): 355-61) and to be cytotoxic to isolated intestinal epithelial cells (Gaginella et al. (1977) J Pharmacol Exp Ther 201(1): 259-66). These features are likely the source of the laxative properties of castor oil which is given as a purgative in humans and livestock (e.g., castor oil is a component of some de-worming protocols because of its laxative properties). In contrast, the methyl ester of ricinoleic acid is ineffective at suppressing water absorption in the hamster model (Gaginella et al. (1975) J Pharmacol Exp Ther 195(2): 355-61).
It has been reported that short- and medium-chain fatty acids and salts (e.g., C6 to C12) have superior fungicidal activity (U.S. Pat. Nos. 5,093,124 and 5,246,716). Not surprisingly, the commercial fungicidal and moss killing product De-Moss comprises mainly fatty acids and salts in this size range. The phytotoxicity of these shorter fatty acids also makes them suitable as broad-spectrum herbicides when used at higher concentrations as is exemplified by the commercial herbicide SCYTHE which comprises the C9 fatty acid pelargonic (nonanoic) acid. U.S. Pat. Nos. 5,093,124, 5,192, 546, 5,246,716 and 5,346,698 teach that C16 to C20 fatty acids and salts such as oleic acid (C18:1) are suitable insecticidal fatty acids. Insecticidal fatty acid products such as M-PEDE and SAFER Insecticidal Concentrate whose active ingredients comprise longer chain fatty acids rich in C16 and C18 components represent real world applications of this scientific information. In contrast, the prior art provides little guidance for the selection of suitable broad-spectrum nematicidal fatty acids and what information exists is often contradictory.
Stadler and colleagues (Stadler et al. (1994) Planta Medica 60(2): 128-132) tested a series of fatty acids against L4 and adult C. elegans and found that a number of common longer chain fatty acids such as linoleic (C18:2), myristic (C14:0), palmitoleic (C16:1) and oleic (C18:1) acids had significant nematicidal activity. C. elegans was not very sensitive to C6 to C10 (medium chain) fatty acids. Stadler et al. commented that their results contrasted with those of an earlier study on the plant parasite Aphelenchoides besseyi where C8 to C12 fatty acids were found to be highly active while linoleic acid—a C18 fatty acid—showed no activity. The differential sensitivity of specific nematodes to various fatty acids is again evident in the study of Djian and co-workers (Djian et al. (1994) Pestic. Biochem. Physiol. 50(3): 229-239) who demonstrate that the nematicidal potency of short volatile fatty acids such as pentanoic acid can vary between species (e.g., Meloidogyne incognita is over a hundred times more sensitive than Panagrellus redivivus). The recent finding by Momin and Nair (Momin & Nair (2002) J. Agric. Food Chem. 50(16): 4475-4478) that oleic acid at 100 μg/mL over 24 hours is not nematicidal to either Panagrellus redivivus or Caenorhabditis elegans further confuses the situation as it directly conflicts with the LD50 of 25 μg/mL (LD90 100 μg/mL) measured by Stadler and coworkers.
In summary, unlike the case for fungicides, herbicides and insecticides, the prior art provides no specific or credible guidance to aid in the selection of suitable nematicidal fatty acids. Moreover, whereas De-Moss, SCYTHE, M-PEDE and SAFER, are examples of successful pesticidal fatty acid products in these three areas respectively, there are currently no examples of commercial nematicidal fatty acid products in widespread use.
Many plant species are reported to be highly resistant to nematodes. The best documented of these include marigolds (Tagetes spp.), rattlebox (Crotalaria spectabilis), chrysanthemums (Chrysanthemum spp.), castor bean (Ricinus communis), margosa (Azardiracta indica), and many members of the family Asteraceae (family Compositae) (Hackney & Dickerson. (1975) J Nematol 7(1): 84-90). In the case of the Asteraceae, the photodynamic compound alpha-terthienyl has been shown to account for the strong nematicidal activity of the roots. Castor beans are plowed under as a green manure before a seed crop is set. However, a significant drawback of the castor plant is that the seed contains toxic compounds (such as ricin) that can kill humans, pets, and livestock and is also highly allergenic. In many cases however, the active principle(s) for plant nematicidal activity has not been discovered and it therefore remains difficult to derive commercially successful nematicidal products from these resistant plants or to transfer the resistance to agronomically important crops such as soybeans and cotton.
Genetic resistance to certain nematodes is available in some commercial cultivars (e.g., soybeans), but these are restricted in number and the availability of cultivars with both desirable agronomic features and resistance is limited. The production of nematode resistant commercial varieties by conventional plant breeding based on genetic recombination through sexual crosses is a slow process and is often further hampered by a lack of appropriate germplasm.
Small chemical effectors can have significant advantages where size exclusion of larger molecules is a concern (e.g., with sedentary plant parasitic nematodes). However, unless the small molecule nematicidal active has high in planta mobility, or the chemical stimulates increased systemic resistance, a transgene encoding an enzyme must still be expressed in an appropriate spatial and temporal manner to be effective. With many plant parasitic nematodes this means that root expression of the nematicidal product is likely important for nematode control. It has been reported that when a constitutive promoter such as a Cauliflower Mosaic Virus (CaMV) 35S promoter is used to drive expression of certain hydroxylase enzymes, no significant amounts of protein production or hydroxylase activity is observed in non-seed tissues (e.g., roots or leaves), nor do hydroxylated fatty acids accumulate (van de Loo et al. (1995) Proc Natl Acad Sci USA 92(15): 6743-7; Broun & Sommerville (1997) Plant Physiol. 113(3): 933-942; Broun et al. (1998) Plant J 13(2): 201-210; U.S. Pat. No. 6,291,742; U.S. Pat. No. 6,310,194).
There remains an urgent need to develop environmentally safe, target-specific ways of controlling plant parasitic nematodes. In the specialty crop markets, economic hardship resulting from nematode infestation is highest in strawberries, bananas, and other high value vegetables and fruits. In the high-acreage crop markets, nematode damage is greatest in soybeans and cotton. There are however, dozens of additional crops that suffer from nematode infestation including potato, pepper, onion, citrus, coffee, sugarcane, greenhouse ornamentals and golf course turf grasses.